MC4WP: Mailchimp for WordPress < 4.12.1 - Author+ Stored XSS via Form Response Messages

Description

The plugin is vulnerable to Stored Cross-Site Scripting via the form response message post meta fields (e.g., 'text_subscribed', 'text_error') due to insufficient input sanitization and output escaping. This makes it possible for authenticated attackers, with Author-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.